1. The Technical Field
The present invention is directed to the field of fiber optics. More particularly, the present invention is directed to an adjustable multi-port optical fiber coupler.
2. The Prior Art
Although four-port fiber optic couplers are known in the art, conventional four-port couplers generally are not readily adjustable. The most common way to fabricate a four-port fiber optic coupler having ports (or legs) A, B, C and D in commercial practice is to: (1) remove a section of the coating from each of two segments of single-mode optical fiber; (2) twist the uncoated fiber sections about each other; (3) heat the fiber sections until they soften; and (4) draw the glass structure axially to reduce the overall fiber diameter. When the overall fiber diameter is sufficiently reduced, some portion of the light propagating in one fiber couples into the other fiber. During the drawing process, light can be propagated in one of the fibers, and the fraction of light coupled to the other fiber can be monitored to precisely control the process, so that couplers with specified coupling performance can be produced. The portions of the first fiber adjacent the uncoated section embody legs A and B of the resulting four-port coupler and the portions of the second fiber adjacent the uncoated section embody ports C and D. Light can be coupled between diagonally opposite legs A and D and/or between diagonally opposite legs B and C of the coupler.
Heating (or cooling) the foregoing structure to a temperature different from the fabrication temperature can change the refractive index of the constituent single mode fibers sufficiently to simultaneously adjust the A/D and C/B coupling or effectively turn the device on or off, i.e., eliminate any fiber-to-fiber coupling. However, maintaining such a device in either the on or off condition, if different from the device fabrication condition, or holding a coupling setting different from that occurring at the device fabrication temperature requires continuous power to maintain the temperature corresponding to the desired condition and/or the desired amount of coupling. Another drawback to this type of coupler is that the A/D and B/C couplings are not independently adjustable.
It would be desirable to provide a multi-port optical fiber coupler that allows efficient and independent adjustment of the respective couplings between pairs of diagonally adjacent legs and which allows efficient and selective complete decoupling without affecting the foregoing adjustments.
The present invention preferably is embodied as a novel four-port fiber optic coupler having legs A, B, C and D that can allow independently adjustable coupling between diagonally opposite legs, for example, legs A/D and C/B, and that can allow for complete decoupling without affecting the foregoing adjustments. That is, the coupler can be adjusted to provide a desired, predetermined amount of coupling between legs A/D and/or C/B. The coupler also can be turned off so that all A/D and C/B coupling is extinguished. The coupler then can be turned back on with the original A/D and/or C/B coupling settings intact.
A coupler according to a preferred embodiment of the present invention includes a continuous fiber segment and first and second injector fibers. The continuous fiber segment has first and second legs A and B, which embody legs A and B of the coupler. The first and second injector fibers embody legs C and D of the coupler.
The injector fibers can include lenses for focusing light to be injected into the continuous fiber segment and to facilitate collecting light to be extracted from the continuous fiber segment. The injector fibers preferably are located on a substrate in optical alignment with respective portions of the continuous fiber segment. In a preferred embodiment, one or more alignment grooves are provided to facilitate this alignment.
In a preferred embodiment, a first depressor is provided to deflect leg A of the continuous fiber segment, thus forming a bend therein, and a second depressor is provided to deflect leg B of the continuous fiber segment, thus forming a bend therein. First and second injector fibers are positioned to direct light at the respective bends thus formed; preferably, the first and second injector fibers are positioned to direct light at the respective transitions from straight to curved fiber. The first and second depressors are independently adjustable. That is, they can be independently manipulated to independently adjust the curvature of the respective bends they impart to legs A and B of the continuous fiber segment. The amount of optical coupling between legs A/D and C/B is a function of the curvature of these bends. In this manner, the first and second depressors can selectively adjust the coupling between diagonally opposite legs of the device.
An adjustable clamp selectively toggles a portion of the continuous fiber segment to selectively extinguish the coupling between the continuous fiber segment and the first and second injector fibers. The coupling can be extinguished by moving the continuous fiber segment out of optical alignment with the first and second injector fibers and/or by relaxing the bends imparted to legs A and B of the continuous fiber segment. The toggling distance can be as little as one fiber diameter or less. Preferably, the foregoing portion of the continuous fiber segment lies in the alignment groove when in the xe2x80x9conxe2x80x9d position. Preferably, the alignment groove is sufficiently deep so that the foregoing portion of the continuous fiber segment remains at least partially within the groove even when toggled to the xe2x80x9coffxe2x80x9d position.
Several couplers according to the present invention can be ganged together and controlled as a unit. That is, the on-off function of several couplers can be tied together, and the first and second depressors of the several couplers can be tied together, so that a single on-off control can be used to turn all of the couplers on or off simultaneously, and so that a single pair of adjusters can adjust the A/D and C/B couplings of all of the couplers simultaneously. Also, several such couplers can be ganged together as a unit, but controlled independently. In yet another embodiment, the depressor settings and clamp can be fixed so as to yield a four-port coupler having fixed coupling properties.
In alternate embodiments, a coupler according to the present invention need not have four ports. For example, the present invention can be embodied as a three-port coupler having legs A, B, and C that can allow adjustable coupling between diagonally opposite legs, for example, legs C and B, and that can allow for complete de-coupling without affecting the foregoing adjustment. Other alternate embodiments are possible, as well.